Coherent manipulation of solid-state spins is important for quantum information processing. Current solid-state spin systems either operate at very low temperatures or are difficult to scale-up. Colloidal quantum dots (QDs), by contrast, can be synthesized in large quantity in solution at low cost, yet with high finesse in size and shape control. Further, they are usually strongly quantum-confined, thus their carriers well isolated from the phonon bath, which could enable long-lived spin coherence at room temperature. We studied coherent spin dynamics in solution-grown lead halide perovskite QDs using transient magneto-optical spectroscopy. We observed ensemble-level quantum beats resulting from an exciton fine-structure gap and quantitatively controlled the gap energy using temperature-programmable lattice distortion. This unique mechanism has important implications for the application of perovskite QDs in quantum light-sources and coherent exciton control. Further, by dissociating excitons using ultrafast interfacial electron transfer, we achieved room-temperature all-optical initialization, manipulation and readout of hole spins in CsPbBr3 QDs. This represents a milestone towards a scalable and sustainable future of spin-based quantum information processing.

This talk will review our investigations of the quantum optical properties of single lead-halide perovskite nanocrystals.

Lead halide perovskites indeed exhibit outstanding optical and electronic properties for a wide range of applications in optoelectronics and for light-emitting devices. The physics of their band-edge exciton, whose recombination is at the origin of the photoluminescence, is particularly rich and at the heart of current research. In particular, the long-lived ground exciton of lead halide perovskite nanocrystals plays a major role in the quantum properties of the emitted light, since it promotes the formation of biexcitons and thus the emission of correlated photon pairs.

On the other hand, the production of single indistinguishable photons with quantum emitters is fundamental to many applications in quantum optics, such as linear optical quantum computing, quantum teleportation and quantum key distribution. Our current investigations thus aim at reducing the dephasing rate and spectral diffusion in these materials and improve the indistinguishability character of the emitted photons, which is investigated via two-photon interference (i.e. the Hong-Ou-Mandel experiment).

Title: Exciton in Halide Perovskite Nanoplatelets: Finite Confinement and Dielectric Effect in Effective Mass Approximation

Kaouther Tlili,†,‡ (Presenting author) Maria Chamarro,‡ Kais Boujdaria,† and Christophe Testelin ‡

†Université de Carthage, Faculté des Sciences de Bizerte, LR01ES15 Laboratoire de Physique des Matériaux : Structure et Propriétés, 7021 Zarzouna, Bizerte, Tunisia.

 ‡Sorbonne Université, CNRS, Institut des NanoSciences de Paris, F-75005, Paris, France.

Two-dimensional (2D) lead halide perovskite (LHP) nanoplatelets (NPLs) have garnered significant interest due to their exceptional optoelectronic properties, such as high exciton binding energy, narrow emission lines, and robust room-temperature excitonic stability. These features make them promising candidates for advanced photonic and optoelectronic applications [1], including light-emitting diodes [2], solar cells [3], and photodetectors [4]. Predicting and controlling excitonic properties in these systems is critical for their integration into practical devices.

In this study, we investigate the excitonic properties of CsPbBr₃​ and CsPbI₃ NPLs using an advanced effective mass approximation (EMA) framework. The model incorporates quantum and dielectric confinements, finite potential barriers, and thickness-dependent carrier masses. Additionally, we explore the dependence of Bloch functions on the NPL width, enabling a detailed understanding of how lattice distortions and confinement modulate the electronic states near the band edges. This refined approach addresses the limitations of infinite-confinement models [5,6,7,8] and provides a realistic description of the excitonic behaviour in nanoscale systems.

Our results reveal a strong influence of dielectric contrast and quantum confinement on excitonic energy and binding energy. For thin NPLs, we achieve good agreement with experimental data [9,10], particularly when finite potential barriers and variable effective masses are included. The model demonstrates a significant enhancement in exciton binding energies due to dielectric effects and quantum confinement. Incorporating Bloch function dependence on NPL width further refines the description of excitonic fine structures, revealing critical interactions between carrier delocalization, dielectric mismatches, and finite potential offsets at interfaces.

This work establishes a robust theoretical framework for understanding and predicting the excitonic properties of LHP NPLs. The findings underscore the influence of the ligand environment and its importance on dielectric and finite confinement effects in achieving precise control over excitonic behaviour in quasi-two-dimensional materials.

References:

[1] Li, H.  et al. (2021). Energy & environmental materials, 4(1), 46-64.

[2] Gan, X et al. (2017). 2D. Solar Energy Materials and Solar Cells, 162, 93-102.

[3] Cui, J et al. (2021). Science Advances, 7(41), eabg8458.

[4] Liu, X et al. (2017). Small, 13(25), 1700364.

[5] Ghribi, A. et al. (2021).. Nanomaterials, 11(11), 3054.

[6] Rajadell et al. (2017). Physical Review B, 96(3), 035307.

[7]  Movilla et al. (2023). Nanoscale Advances, 5(22), 6093-6101.

[8] Gramlich, M et al. (2022). Advanced Science, 9(5), 2103013.

[9] Bohn, B. J et al. (2018). Nano letters, 18(8), 5231-5238.

[10] Wang, S et al. (2022).

At the center of quantum technology lies the light-matter interactions. Chiral quantum optics is a rising field of research in which the momentum- and spin-dependent asymmetric light-matter interaction offers novel utilization of photonic and electronic degrees of freedom. However, it remained challenging to generate and understand the chiral photon emission in a controlled manner since the physical or structural chirality does not always translate into optical chirality or spin angular momentum (SAM). Here, we report an efficient generation of chiral quantum light in single perovskite quantum dots (PQDs), which are one of the brightest quantum emitters known so far but at the same time are intrinsically achiral.^[1] By coupling the PQD with chiral plasmonic particles (Au helicoid),^[2] we prompted the emission of single photons with more than an order of magnitude increased degree of circular polarization (DOCP) and Purcell factor of 2-3. Optical helicity of localized plasmon was responsible for the chirality transfer, which was largely dictated by the 3-dimensional structure-defined surface current flow of Au particle. Critical prerequisites for chiral light generation were to finely tune the PQD emitting energy to resonate with optical helicity and at the same time detune from the localized surface plasmon resonance (LSPR) band of the particle. Further, the emission handedness was retained constant under varying excitation conditions, while the photon flux of chiral emission was controlled by the excitation energy near and far above the band edge, which flipped the sign of excitation dissymmetry factor. These results shed light on the largely unexplored mechanism of chiral light-matter interaction and promises the deterministic utilization of chiral quantum light source.

Colloidal perovskite quantum dots (PQDs) are an exciting platform for on-demand quantum, and classical optoelectronic and photonic devices. However, their potential success is limited by the extreme sensitivity and low stability arising from their weak intrinsic lattice bond energy and complex surface chemistry. Here we report a novel platform of buried perovskite quantum dots (b-PQDs) in a three-dimensional perovskite thin-film, which overcomes surface related instabilities in colloidal perovskite dots. The b-PQDs demonstrate ultrabright and stable single-dot emission, with resolution-limited linewidths below 130 μeV, photon-antibunching (g²(0)=0.1), no blinking, suppressed spectral diffusion, and high photon count rates of 10⁴/s, consistent with unity quantum yield. The ultrasharp linewidth resolves exciton fine-structures (dark and triplet excitons) and their dynamics under a magnetic field. Additionally, b-PQDs can be electrically driven to emit single photons with 1 meV linewidth and photon-antibunching (g²(0)=0.4). These results pave the way for on-chip, low-cost single-photon sources for next generation quantum optical communication and sensing.

Solution-processable semiconductors like halide perovskites and certain molecules are promising for next-generation spin-optoelectronic applications [1]. Yet, we don’t fully understand what governs spin and light polarization in these materials, and even less how these are affected by chirality.[2]

In this talk, I will give an overview of our recent efforts to understand the spin-optoelectronic performance of these materials through time-, space- and polarization-resolved spectroscopy and microscopy.

For investigating halide perovskite films, we pushed broadband circular dichroism to diffraction-limited spatial and 15 fs time resolution for creating a spin cinematography technique to witness the ultrafast formation of spin domains due to local symmetry breaking and spin-momentum locking [3].

I will then briefly explain the fundamentals and artefacts involved in measuring circularly polarized luminescence reliably and introduce an open-access methodology and code to do so [4]. Finally, I will show our most recent development of a transient sensitive broadband full Stokes-vector spectroscopy with unprecedented time- and polarization resolution to track the emergence of chiral light emission [5].

[1] Nature Reviews Materials 8, 365 (2023)

[2] Nature Reviews Chemistry, in press (2024/25)

[3] Nature Materials 22, 977 (2023)

[4] Advanced Materials 35, 2302279 (2023)

[5] unpublished (2024/25).

Perovskite quantum dots are attractive semiconductor nano-objects for quantum light emission. Although initial electronic and excitonic fine structure calculations were reasonably predictive for quantum confinement and magnetic field effects, questions related to the coupling of the quantum confined excitonic states to the lattice are still open. This presentation will give some insights on electron coupling to the lattice, excitonic polarons and lattice polymorphism in the 3D halide perovskites used as active materials in perovskite quantum dots. New results on the empirical modelling of Fröhlich excitonic polarons in the context of coupling to multiple phonons will be given. The refinement of empirical excitonic polaron models necessary to fully account for the variation of the exciton barycenter and reduced masses as a function of the coupling parameters will be described, The crossover from the weak coupling regime to the medium coupling regime for excitonic polarons will be quantified by the variations of  the virtual phonon populations, effective dielectric constants, barycenter and reduced masses and Huang-Rhys factors. The connection with  empirical modelling of free polarons will be proposed using Lee-low and Pines theory to define exciton binding energies and to stress the interplay between populations of virtual phonons and electron-hole correlations.

Semiconducting perovskites hold great promise for demanding applications involving light emission, such as lasers. In this respect, fully inorganic colloidal CsPbBr3 nanoplateltes (NPLs) could be of interest since somewhat similar colloidal CdSe NPLs showed exceptionally high material gain related to stimulated emission through the biexciton-exciton transition[1].

Here, we report on a spectroscopic study of CsPbBr3 2D NPLs, geared towards understanding the photo-excited states and evaluating the performance metrics related to optical gain. In line with literature reports, we show that photo-excitation of 2D CsPbBr3 NPLs leads to the formation of strongly bound excitons. Importantly, the pronounced difference between the oscillator strength of the ground-state-to-exciton transition in absorption and emission points towards considerable exciton localization, for which we estimate a coherence area of a mere 1-2 nm². Furthermore, we show that this localization comes with the formation of an exciton-polaron, which shifts the exciton emission to the red of the exciton absorption.

Through pump-probe spectroscopy using co-polarized and cross-polarized pump and probe beams, we demonstrate that light absorption in the presence of excitons leads to bound biexcitons. These biexcitons have singlet character, and the biexciton emission exhibits an additional Stokes shift with respect to the biexciton absorption. Furthermore, the oscillator strength of this transition indicates that the biexciton area is more than 5 times smaller than the cross-sectional area of the exciton. Both elements suggest that like excitons, biexcitons couple with the CsPbBr3 lattice to form strongly localized biexciton polarons.

After fs optical pumping, CsPbBr3 exhibit a strongly redshifted, weak and short-lived band of net stimulated emission. Using the exciton and biexciton characteristics, we can account for this transient optical gain by assuming stimulated emission across the biexciton-exciton transition. We relate the limited gain performance – as compared to, for example, CdSe NPLs – to the pronounced localization of the biexciton or biexciton polaron.

Colloidal semiconductor nanocrystals are a candidate source of time-correlated and entangled photons through the cascaded radiative relaxation of multiexcitonic states (multiple excitons within the same nanocrystal). However, the efficient nonradiative Auger-Meitner decay of multiexcitons renders them mostly nonemissive. This limits not only potential uses but also their investigation, particularly their spectroscopy. I will present the heralded spectroscopy of three-photon cascades from triexcitons in giant CsPbBr₃ nanocrystals at room temperature.[1] Heralded spectroscopy, realized previously in our group,[2] is a single-particle technique using a single-photon sensitive, 180-picosecond time-resolved spectrometer based on a single-photon avalanche diode (SPAD) array. By post-selecting events of triple photon detections following a single laser pulse, we isolate the triexciton relaxation cascades. This allows us to resolve the weak binding energies associated with the triexciton and the biexciton emissions in the cascade (1.13 ± 0.27 and 0.46 ± 0.28 meV, respectively). Simultaneously, we measure the lifetime of each relaxation step individually, despite their high similarity (triexciton: 0.51 ± 0.08 ns, biexciton: 0.82 ± 0.11 ns, and exciton: 2.21 ± 0.17 ns). These nanocrystals also exhibit near unity values of the second- and third-order correlation functions, g⁽²⁾(0) = 0.97 ± 0.01 and g⁽³⁾(0,0) = 0.94 ± 0.02. Those weak exciton–exciton interactions are in accordance with the nanocrystals’ diameter of 26 nm, more than four times the exciton Bohr diameter in CsPbBr₃ of 7 nm,[3] whereas stronger interaction and binding were previously found in smaller-size (6 nm edge) CsPbBr₃ nanocrystals by our group, studying biexcitons using the same technique.[4] We also combine fluorescence lifetime analysis, photon statistics, and spectroscopy, to verify emission from a single emitter despite the high emission quantum yields of multiply excited states and the comparable emission lifetimes and energies of singly and multiply excited states. I will also compare the biexciton and exciton emission properties during fluctuations in the nanocrystal’s emissivity (“blinking” between on, gray, and off states). Such blinking is typically attributed to Auger-Meitner recombination with excess charges or to surface-trap recombination. I will present indications that blinking can change the multiexciton-to-exciton emission rate ratio, which could be a potential pathway toward control of the photon number statistics of multiexcitonic emission cascades. Finally, I will discuss how this effect changes in nanocrystals of different sizes.

Colloidal perovskite nanocrystals have emerged as promising candidates for next-generation quantum light sources based on their excellent optical properties. In this context, the method to optically prepare the emitting states is a critical step. While previous works on epitaxial quantum dots have stablished that resonant excitation can improve the performance of quantum emitter when compared to above-bandgap excitation by avoid the dephasing processes involved in hot carrier cooling, low-background resonant excitation of a single colloidal perovskite nanocrystal has not yet been achieved. Here, we demonstrate resonant three-photon excitation (3PE) of a single perovskite nanorod at both room temperature and cryogenic temperature. Third-harmonic generation, arising as a background created from the substrate interface is observed and further eliminated using polarization by a factor of 10⁵, resulting in a signal to background ratio larger than 100. The giant three-photon absorption cross-section at room temperature is measured to be around 10^-76 cm⁶·s²·photon^-2 at resonance, which is two orders of magnitude larger than those of traditional CdSe-based nanocrystals. Additionally, similar fluorescence and blinking properties between upper-bandgap 1PE and resonant 3PE are observed in room temperature, highlighting the sample’s photostability under high power NIR beams. This low-background resonant three-photon excitation method has great potential to improve the optical coherence properties of perovskite nanocrystal.

Metal halide perovskites possess unique optical and electronic properties, including high light absorption, high mobility, tuneable bandgaps and high defect tolerance, and these properties make them very promising for achieving high performance optoelectronic devices. Here we demonstrate several examples of using perovskites as emitters for applications in multifunctional displays, visible light communications (VLCs) and quantum random number generators.

Firstly, we develop multifunctional displays using highly photo-responsive metal halide perovskite LEDs (PeLEDs) as pixels. With efficient defects passivation of perovskite layers, the red emissive PeLEDs shows an external quantum efficiency (EQE) of around 10% when working at LED model and a power conversion efficiency (PCE) of 5.34% at photovoltaic model. Due to the strong photo response of the PeLED pixels, the display can be simultaneously used as touch screen, fingerprint sensor, ambient light sensor, and image sensor without integrating any additional sensors. In addition, decent light-to-electricity conversion efficiency of the pixels also enables the display to act as a photovoltaic device which can charge the equipment.^[1] The multiple-functions of our PeLED pixels can not only simplify the display module structure and realize ultra-thin and light-weight display, but also significantly enhance the user experience by these advanced new applications, and this is a feature hardly possible for conventional LED technologies.^[2]

Additionally, we further demonstrate an all perovskite based visible light communication (VLCs) system with PeLEDs and PePVs, and quantum number random generator based on PeLEDs.^[3] These demonstrations show that perovskite based optoelectronics will have special advantages in the future for high performance and low-cost telecommunication devices.

The ability to control the emission of individual nanocrystals through external electric fields has garnered significant interest in the field of quantum technology. Among the most promising materials for quantum emitters are colloidal lead halide perovskite nanocrystals due to near unity quantum yields and their flexibility in size, shape, and composition engineering. An external electric field induces the quantum-confined Stark Effect (QCSE), which is expected to result in a reduced overlap of the electron and hole wavefunction and an energy shift of the bandgap. This allows to extract polarizability and permanent dipole moment, the latter affecting, e.g., the Rashba effect in lead halide perovskite nanocrystals. Moreover, the field-induced tuning of the excitonic fine structure[1] in single nanocrystals may pave the path for single or entangled photon emitters.

Here, we present an approach of polarization-resolved photoluminescence (PL) spectroscopy on highly anisotropic, single CsPbBr₃ nanorods (NRs) for exploring the QCSE under a well-defined angle between crystal axes and electric field[2]. A polarizability of 23 meV/(MV/cm)² and a permanent dipole of 6.4*10^-30 Cm could be extracted, e.g., for a single CsPbBr₃ NR aligned at 65° relative to an applied electric field of up to ± 333 kV/cm. We observe a clear correlation between energy shift, spectral line width, and PL intensity under the influence of the QCSE. A detailed analysis on a statistically relevant number of NRs revealed an interesting correlation between PL intensity and polarizability: The higher the PL intensity, the lower the polarizability. Comparing aged NRs with low PL intensities with freshly synthesized ones shows that 44% of the aged NRs reveal a polarizability larger than our error bar (i.e., > 3 meV/(MV/cm)², whereas only 7% of the fresh NRs show a similar finding. Our work thus highlights new insights and challenges in terms of modulating single lead halide perovskite nanocrystals in directional electric fields.

Generating and manipulating non-classical light in the form of a stream of single photons is central to a broad range of emerging quantum-light applications, from quantum computing to quantum sensing and quantum imaging. In this talk, I will present recent advances in using colloidal lead-halide perovskite quantum dots (QDs) towards this end, with advantageous attributes such as spectral tunability, solution-processability, and scalability.

Sharing results from single-particle spectroscopy and ab-initio molecular-dynamics simulations, I will argue that the pronounced exciton-phonon coupling in perovskite QDs[1] is key to understanding and manipulating (multi)excitons in these materials. To illustrate this point, I will elaborate on the strikingly different photophysics in individual perovskite QDs at cryogenic and room temperature. At cryogenic temperature, perovskite QDs behave as textbox semiconductors, capable of cavity-free coherent quantum-light emission and hosting phenomena such as single-photon superradiance[2] and superabsorption.[3] The latter two concepts have both been proposed theoretically early on but evaded experimental demonstration until very recently. At room temperature, on the other hand, coupling of the exciton to large-amplitude lattice vibrations in the QD core and at the QD surface leads to pronounced emission broadening[4] and localization of the exciton wavefunction.[5] While the latter inevitably accelerates thermally activated decoherence processes, it can auspiciously be leveraged to increase single-photon purity, up to 98% for cavity-free, nonresonantly excited single perovskite QDs at room temperature.[6]

References:

[1] C. Zhu, et al., Adv. Optical Mater. 2024, 12, 2301534.

[2] C. Zhu, et al., Nature 2024, 626, 535–541.

[3] S.C. Boehme, et al., under review.

[4] G. Rainò, et al., Nat. Commun. 2022, 13, 2587.

[5] L.G. Feld, at al., DOI: 10.48550/arXiv.2404.15920.

[6] C. Zhu, et al., Nano Lett. 2022, 22, 3751–3760.

Lead halide perovskite quantum dots (PQDs) have emerged as promising materials for advanced optoelectronic applications due to their tunable excitonic properties.[1] However, achieving strong light-matter coupling in PQD films has been challenging due to issues with film quality, weak oscillator strengths and spectral diffusion.[2] Leveraging advances in PQD synthesis and film preparation,[3] we present the successful formation of uniform, thick, scattering-free films with well-defined excitonic transitions. By coupling these highly transparent cesium lead bromide (CsPbBr₃) PQD solids with resonant modes of metallic resonators, multiple cavity exciton-polaritons at room temperature are obtained, evidenced by significant alterations in the absorption and emission spectra.[4]

Unlike traditional PQD systems, the dynamics observed by transient absorption spectroscopy (TAS) is dominated by the interplay of polaritonic states with dark-state reservoirs, while effects such as polaron formation are seemingly absent. The study also reports a substantial reduction in photoemission linewidth and ultrafast modulation of optical absorption properties on the picosecond timescale. These insights establish the groundwork for developing polaritonic devices with tunable photophysical properties and lay the foundation for pursuing phenomena like Bose-Einstein condensation in solid-state systems.[4]